Various systems exist for assisting a crew in the piloting of an aircraft, notably during an approach phase. Particularly well known among these systems are the flight management systems, called FMS, schematically represented in FIG. 1 and comprising the following functions:                location LOCNAV, identified 1: making it possible to locate the aircraft by means of various geolocation tools or instruments (GPS, GALILEO, VHF radio beacons, inertial units),        flight plan FPLN, identified 2: making it possible to input the geographic elements forming the skeleton of the route to be followed (departure and arrival procedures, way points, etc.),        navigation database NAVDB 3: making it possible to construct geographic routes and procedures from data included in the bases (points, beacons, interception or altitude legs, etc.),        performance database PRF DB 4: containing the aerodynamic and engine parameters of the craft,        lateral trajectory TRAJ 5: making it possible to construct a continuous trajectory from points in the flight plan, observing the aeroplane performance levels and the containment constraints,        predictions PRED 6: making it possible to construct an optimized vertical profile compatible with the lateral trajectory,        guidance GUIDANCE 7: making it possible to guide, in the lateral and vertical planes, the aircraft on its 3D trajectory, while optimizing the speed,        digital data link DATALINK 8: making it possible to communicate with the control centres, the airlines and the other aircraft.        
In a typical approach phase as represented in FIGS. 2a, 2b and 2c, an aircraft 9 seeks to follow a target trajectory 10 to reach a landing runway 11. Generally, an approach phase comprises a first part consisting of one or more “linear” sections 12 followed by a final “angular” section 13 converging towards a touchdown point 14, generally situated close to the runway threshold 11. The final angular section 13 is possibly followed by one or more linear sections 12 in the case of a go-around phase for an interrupted approach, also referred to as “Missed Approach”. On a linear section 12 as represented in FIG. 2b, a linear deviation 15 represents the distance separating an estimated position 16 of the aircraft 9 and a desired position 17 on the target trajectory 10. The linear deviation 15 can be expressed by a lateral component and a vertical component.
On an angular section 13 as represented in FIG. 2c, an angular deviation 18 represents the angle formed at the touchdown point 14, between the target trajectory 10 and a straight line D1 joining the touchdown point 14 to the estimated position 16 of the aircraft 9. The angular deviation 18 can be expressed by a lateral component and a vertical component.
In the known systems, the aim is to minimize the linear deviation on a linear section. Along a linear section produced with a navigation performance requirement, or RNP, standing for “Required Navigation Performance”, the requirement is to keep the linear deviation 15 below a limit value. When the difference exceeds the limit value, the system provides for alerting the crew to enable it to decide on the corrective measures to be performed.
Along a linear section, the known systems propose to the crew, by means of a human-machine interface, or HMI, graphic representation means, commonly called “monitoring” means, that make it possible to monitor the navigation performance. In particular, the known systems propose a monitoring of the navigation performance on a linear section conforming to a current standardization, in particular the standard ICAO PBN Manual, doc 9613.
According to this standardization, a linear deviation 15 is graphically represented, as schematically represented in FIG. 3, on a first lateral deviation axis 21 and a second vertical deviation axis 22. This is called linear monitoring 20, the representation of a linear deviation 15, expressed laterally 23 and vertically 24, on the two deviation axes 21 and 22. The linear deviation 15 is represented on the lateral deviation axis 21 according to a lateral scale 25, called RNP, and on the vertical deviation axis 22 according to a vertical scale 26, called V-RNP, by means of a cross 27 symbolically representing the aircraft 9.
According to this standardization, when the aircraft 9 is positioned on the target trajectory 10, it appears centred on each of the scales, lateral 25 and vertical 26; a deviation thus being able to be positive or negative for each of the deviation axes 21 and 22. The equivalent distance between two RNP or V-RNP graduations is variable, for example according to the approach phases, the linear sections, the flight conditions or the aircraft type. Typically, in a navigation performed with a required navigation performance RNP, an alert will be transmitted to the crew when a linear deviation greater than 2 RNP graduations occurs.
As an example, in an approach phase, a monitoring of the navigation performance of “RNP 0.3” type may be required. The distance between two RNP graduations is then equal to 0.3 nautical miles, and the lateral linear deviation should be centred on the trajectory with a maximum tolerance of plus or minus 1 nautical mile, corresponding to plus or minus 2 RNP, a threshold from which an alert is transmitted to the crew. It will be recalled that a nautical mile, also called NM, is a unit commonly used by the person skilled in the art in the aeronautical field, 1 nautical mile corresponding to 1852 meters. The International Civil Aviation Organization (ICAO) defines standards at international level; in particular, the RNP values of 4 NM, 1 NM, 0.3 NM or 0.1 NM are the reference values used worldwide. The principle of the navigation assistance method according to the invention applies however to any RNP value.
On the final “angular” section, the aim is to minimize the angular deviation 18. In the known systems, a transmitting beacon arranged in proximity to the threshold of the landing runway 11 embodies the touchdown point 14. The reception by the aircraft 9 of the signal transmitted by the beacon then makes it possible to determine the angular deviation 18 of the aircraft 9 relative to its target trajectory 10. Thus, the expression ILS (Instrument Landing System) navigation applies, for an approach performed on an angular section in which the aim is to minimize the angular deviation 18. The navigation assistance method applies also to other types of angular approaches, such as, for example, the MLS (Microwave Landing System) approaches which rely on a wireless transmitting beacon, or, for example, the FLS (FMS Landing System) approaches which rely on a virtual beacon.
In the systems currently implemented, nothing is defined to guarantee the maintenance of navigation performance on an angular section. The systems do not propose any sophisticated monitoring tool to enable the crew to control the descent along an angular section, and have sufficient reaction time to manoeuvre the aircraft, in particular as the approach continues and the cone of the deviations shrinks.
Moreover, the switchover between the two types of navigation, from linear to angular (and from angular to linear in the case of an interrupted approach), is performed with no particular management of the transition. It is possible to ensure the monitoring of the navigation performance on the linear section, then when the aircraft 9 enters into the angular section, the linear monitoring is interrupted, the crew observes the angular deviation and decides on the corrective measures to be applied, without the possibility of anticipation at the time of the transition.